LWIR imaging lens, image capturing system having the same, and associated method

ABSTRACT

An imaging lens for use with an operational waveband over any subset of 7.5-13.5 μm may include a first optical element of a first high-index material and a second optical element of a second high-index material. At least two surfaces of the first and second optical elements may be optically powered surfaces. A largest clear aperture of all optically powered surfaces may not exceed a diameter of an image circle of the imaging lens corresponding to a field of view of 55 degrees or greater by more than 30%. The first and second high-index materials may have a refractive index greater than 2.2 in the operational waveband, an absorption per mm of less than 75% in the operational waveband, and an absorption per mm of greater than 75% in a visible waveband of 400-650 nm.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation application based on pending application Ser. No.13/356,211, filed Jan. 23, 2012, the entire contents of which is herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments relate to an imaging lens for the long wavelength infrared(LWIR) region, an image capturing system including the same, andassociated methods.

2. Description of the Related Art

As with most technology, there is a demand for smaller and cheaperthermal imagers, whether as stand alone devices or integrated intomobile devices, electronic device, and so forth.

SUMMARY OF THE INVENTION

Embodiments are directed to an imaging lens for use with an operationalwaveband over any subset of 7.5-13.5 μm. The imaging lens may include afirst optical element of a first high-index material, the first opticalelement having a front surface and a rear surface and a second opticalelement of a second high-index material, the second optical elementhaving a front surface and a rear surface, the front surface of thesecond optical element facing the rear surface of the first opticalelement. At least two surfaces of the first and second optical elementsmay be optically powered surfaces. A largest clear aperture of alloptically powered surfaces may not exceed a diameter of an image circleof the imaging lens corresponding to a field of view of 55 degrees orgreater by more than 30%. The first and second high-index materials mayhave a refractive index greater than 2.2 in the operational waveband, anabsorption per mm of thickness less than 75% in the operationalwaveband, and an absorption per mm of thickness greater than 75% in avisible waveband of 400-650 nm.

The first and second high-index materials may be identical.

At least one of the first and second high-index materials may besilicon.

The largest clear aperture does not exceed the diameter of the imagecircle by more than 20%.

Three surfaces of the first and second optical elements may be opticallypowered surfaces.

The optically powered surfaces may be the front and rear surfaces of thefirst optical element and the front surface of the second opticalelement.

All three of the optically powered surfaces may be aspheric.

Each optically powered surface may have a positive power at an apexthereof.

Each optically powered surface may have a maximum sag height differenceacross the clear aperture of less than 100 μm.

Each optically powered surface may have a maximum sag height differenceacross the clear aperture of 50 μm or less.

One, two, or three of the optically powered surfaces may be aspheric.

The F-number of the imaging lens may be less than 1.1.

The imaging lens may include an optical stop at the front surface of thefirst lens element.

The optical stop may be effectively in contact with the front surface ofthe first lens element.

The optical stop may include a metal, e.g., chromium, apertureeffectively in contact with the front surface of the first lens element.

The metal aperture may have a thickness of less than 200 nm.

Transmission through the optical stop may be less than 0.5% in theoperational waveband.

The optical stop may be adhered to the front surface of the firstoptical element.

Center thicknesses of the first and second optical elements may bewithin 15% of one another.

A center thickness of each of the first and second optical elements isgreater than 500 μm and less than 1500 μm, e.g., greater than 500 μm andless than 1000 μm.

The imaging lens may include a spacer between and adhered to the firstand second optical elements.

The imaging lens may include a first flat region on the rear surface ofthe first optical element and a second flat region on the front surfaceof the second optical element, wherein the spacer is adhered to thefirst and second optical elements at the first and second flat regions.

The imaging lens may include a diffractive optical element on the frontsurface of the first optical element, the rear surface of the firstoptical element, the front surface of the second optical element, and/orthe rear surface of the second optical element.

The diffractive optical element may be on an optically powered surfacehaving the greatest power.

Embodiments are directed to an imaging system for use with anoperational waveband over any subset of 7.5-13.5 μm. The imaging systemmay include a sensor for use with an operational waveband over anysubset of 7.5-13.5 μm and an imaging lens imaging the operationalwaveband onto the sensor. The imaging lens may include a first opticalelement of a first high-index material, the first optical element havinga front surface and a rear surface and a second optical element a secondhigh-index material, the second optical element having a front surfaceand a rear surface, the front surface of the second optical elementfacing the rear surface of the first optical element. At least twosurfaces of the first and second optical elements may be opticallypowered surfaces. A maximum clear aperture of all optically poweredsurfaces may not exceed an image diagonal of the sensor by more than30%. The first and second high-index materials may have a refractiveindex greater than 2.2 in the operational waveband, an absorption per mmof thickness less than 75% in the operational waveband, and anabsorption per mm of thickness greater than 75% in a visible waveband of400-650 nm.

A ratio of an optical track length of the imaging system to an imagediagonal of the sensor may be less than 2.5.

The sensor may include a cover glass of a third high-index materialhaving a refractive index greater than 2.2 in the operational band.

The third high index material and at least one of the first and secondhigh index materials may be identical.

The third high index material may be silicon.

The cover glass has a thickness greater than 0.5 mm and less than 1.0mm.

A distance between an apex of the rear surface of the first opticalelement and an apex of the front surface of the second optical elementmay be less than 50% greater than a distance between an apex of the rearsurface of the second optical element and the cover glass.

A distance between an apex of the rear surface of the first opticalelement and an apex of the front surface of the second optical elementmay be greater than 50% larger than a distance between an apex the rearsurface of the second optical element and the cover glass.

The imaging system may include an adjustment mechanism for altering adistance between the imaging lens and the sensor.

The adjustment mechanism may include a threaded barrel assembly housingthe imaging lens.

Embodiments are directed to an electronic device including an imagingsystem for use with an operational waveband over any subset of 7.5-13.5μm.

Embodiments are directed to an imaging lens for use with an operationalwaveband over any subset of 7.5-13.5 μm. The imaging lens may include afirst optical element of a first high-index material, the first opticalelement having a front surface and a rear surface, an first opticallypowered surface on one of the front and rear surfaces of the firstoptical element, and a second optical element of a second high-indexmaterial, the second optical element having a front surface and a rearsurface, the front surface of the second optical element facing the rearsurface of the first optical element, a second optically powered surfaceon one of the front and rear surfaces of the second optical element. Thefirst and second high-index materials have a refractive index greaterthan 2.2 in the operational waveband, an absorption per mm of thicknessless than 75% in the operational waveband, and an absorption per mm ofthickness greater than 75% in a visible waveband of 400-650 nm.

The first optically powered surface may be on the front surface of thefirst optical element and the second optically powered surface may be onthe rear surface of the second optical element.

The rear surface of the first optical element and the front surface ofthe second optical element may have negligible optical power therein.

Embodiments are directed to an imaging lens for use with an operationalwaveband over any subset of 7.5-13.5 μm. The imaging lens may include afirst silicon optical element, the first silicon optical element havinga front surface and a rear surface; and a second silicon opticalelement, the second silicon optical element having a front surface and arear surface, the front surface of the second silicon optical elementfacing the rear surface of the first silicon optical element. At leasttwo surfaces of the first and second optical elements may be etchedoptically powered surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages will become more apparent tothose of ordinary skill in the art by describing in detail exemplaryembodiments with reference to the attached drawings, in which:

FIG. 1 illustrates a schematic side view of an imaging capturing systemin accordance with an embodiment;

FIG. 2 illustrates a schematic side view of an imaging capturing systemin accordance with an embodiment;

FIGS. 3A to 3C illustrate plots of lens sag and slope versus radialaperture for lens surfaces having power therein in FIG. 2;

FIG. 4 illustrates a schematic side view of an image capturing system inaccordance with an embodiment;

FIG. 5 illustrates a schematic side view of an image capturing system inaccordance with an embodiment;

FIG. 6 illustrates a cross-sectional view of a module assembly includingan imaging system in accordance with an embodiment;

FIG. 7 illustrates a schematic perspective view of a computerincorporating an image capturing device in accordance with embodiments;and

FIG. 8 illustrates a schematic perspective view of a mobile deviceincorporating an image capturing device in accordance with embodiments.

DETAILED DESCRIPTION

Example embodiments will now be described more fully hereinafter withreference to the accompanying drawings; however, they may be embodied indifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete.

In designing long wavelength infrared (LWIR) sensors, also known asthermal imagers, materials for use as thermal lenses typically have hightransmission in the LWIR waveband of 7.5-13.5 μm. Current typicalmaterials for thermal lenses include germanium (Ge), chalcogenide glass,zinc selenide (ZnSe), and zinc sulfide (ZnS). However, many opticalmaterials having other desirable properties are excluded due to a highabsorption in the LWIR waveband of 7.5-13.5 μm.

As described in detail below, as designs for LWIR sensors shrink, e.g.,for use in mobile devices, a thickness of material used for thermallenses may decrease sufficiently to allow materials that are typicallyconsidered too absorptive in the LWIR waveband to be used as thermallenses. This allows the use of other materials, e.g., silicon, that havea strong absorption band in the LWIR waveband, but offer otheradvantages, e.g., manufacturability, low coefficient of thermalexpansion, low dispersion, etc., to be employed.

The imaging lenses discussed in detail below are to be operational overany subset of the LWIR waveband. These imaging lenses are designed to bemade in a high index material, i.e., greater than 2.2, having anabsorption per mm of thickness less than 75% in the operationalwaveband, and an absorption per mm of thickness greater than 75% in avisible waveband of 400-650 nm. While silicon meets these parameters andprovides advantages noted above, other materials that meet theseparameters may also be used.

FIG. 1 illustrates a schematic side view of an image capturing system150 in the LWIR waveband in accordance with an embodiment. Asillustrated in FIG. 1, the image capturing system 150 includes animaging lens 100 and a sensor 130.

The imaging lens 100 may include a first optical element 110 and asecond optical element 120. In the schematic illustration of FIG. 1, aspacer (which would include surfaces C and D, see FIG. 6) between thefirst optical element 110 and the second optical element 120 has beenomitted for clarity.

In this particular embodiment, both the first optical element 110 andthe second optical element 120 are planoconvex lenses. A surface A, herean input surface of the imaging lens 100, of the first optical element110 and a surface F, here a final surface of the imaging lens 100, bothhave optical power. One or both of these surfaces may be aspheric.Surface B of the first optical element 110 and surface E of the secondoptical element 120 have no optical power, here are both planar, andface each other.

The imaging lens 100 may also include an aperture stop 102. For example,the aperture stop 102 may be adjacent surface A, e.g., directly onsurface A, of the first optical element 110. The aperture stop 102 maybe made of metal, e.g., chromium, a dyed polymer, or any suitablematerial that is opaque to LWIR. The aperture stop 102 may be at anyappropriate location within the imaging lens 100. The aperture stop 102may be thin, e.g., have a thickness of less than 200 nm, but thickenough to be effective, i.e., have a transmission therethrough of lessthan about 0.5% in the operational waveband. The f-number for theimaging lens 100 may be less than 1.1.

If the material used for one or both optical elements 110, 120 presentschromatic dispersion over an operational waveband or if the imaging lens100 otherwise requires correction, a diffractive element 104 may beprovided on one or more of the surfaces A, B, E, or F. For example, thediffractive element 104 may be on the surface having the most opticalpower, here, surface F.

The sensor 130 may include a sensor cover glass 132 and pixels in asensor image plane 134, the pixels detecting LWIR radiation. The sensorcover glass 132 may be made of silicon and may have a thickness between0.5 mm and 1.0 mm. The working distance of the image capturing system150 is a distance from a bottom surface, i.e., an apex of the bottomsurface, of the imaging lens 100, here surface F, to a top surface ofthe cover glass 132. The optical track length of the imaging capturingsystem 150 is a distance from an apex of the first surface of theimaging lens 100, here surface A, to the sensor image plane 134.

While the above embodiment provides a design in which only two surfaceshave optical power for the imaging lens 100 along the z-direction, themaximum clear aperture of the imaging lens 100 (here at surface F) ismuch larger, e.g., more than 50% greater, than the sensor imagediagonal, i.e., a diagonal across the sensor image plane 134, and themaximum SAG of the imaging lens 100 (also at surface F) is relativelylarge, e.g., much greater than 100 μm. In the particular designillustrated in FIG. 1, the maximum clear aperture is 2.6 mm, the sensorimage diagonal is 1.7 mm, and the maximum SAG is 203 μm.

However, having the maximum clear aperture being much larger than thesensor image diagonal and having a large maximum SAG may presentmanufacturability and cost issues, particularly when these opticalelements are to be made on a wafer level, as described later. Withoutreference to a particular sensor, i.e., the sensor image diagonal, themaximum clear aperture may be defined relative to an image circle of thelens. In particular, the image circle of the lens is to be understood asthe diameter of the image produced at the focal plane of the lenscorresponding to a given field of view (FOV), e.g., 55 degrees orgreater, of the lens. In the context of an imaging system having animaging lens and an image sensor, the image circle is understood to bethe largest distance across the image that is used by the image sensor,typically the image sensor diagonal of the sensor with which the imaginglens used or intended to be used.

Therefore, embodiments illustrated in FIGS. 2 to 5 may employ a twooptical element design in which optical power is provided on threesurfaces. Spreading the optical power over three surfaces, whileincreasing the number of surfaces to be manufactured, allows a maximumclear aperture much closer in size to the sensor image diagonal (orimage circle) and a reduced SAG to be realized. In embodiments, themaximum clear aperture of the imaging lens may be less than 30% greater,e.g., less than 20% greater, than the sensor image diagonal or the imagecircle corresponding to a FOV of 55 degrees or greater.

FIG. 2 illustrates a schematic side view of an imaging capturing system250 in the LWIR waveband in accordance with an embodiment. Asillustrated in FIG. 2, the image capturing system 250 includes animaging lens 200 and a sensor 230.

The imaging lens 200 may include a first optical element 210 and asecond optical element 220. In the schematic illustration of FIG. 2, aspacer (which would include surfaces C and D, see FIG. 6) provides anair gap between the first optical element 210 and the second opticalelement 220 has been omitted for clarity. Features outside the opticalsurfaces could be used to nest them together, e.g., the air gap may beprovided by a barrel or housing.

In this particular embodiment, three surfaces, here surfaces A, B, andE, have optical power therein. One, two, or all three surfaces may beaspheric. All three surfaces may have a positive power at the apexthereof, i.e., may be convex at the apex thereof. The imaging lens 200may also include the aperture stop 202, which may have the sameconfiguration/properties noted above for aperture stop 102. The f-numberfor the imaging lens 200 may be less than 1.1.

If the material used for one or both optical elements 210, 220 presentschromatic dispersion over an operational waveband or if the imaging lens200 otherwise requires correction, a diffractive element 204 may beprovided on one or more of the surfaces A, B, E, or F. For example, thediffractive element 204 may be on the surface having the most opticalpower, here, surface E.

The sensor 230 may include a sensor cover glass 232 and pixels in asensor image plane 234, the pixels detecting LWIR radiation. In theparticular configuration, the sensor image diagonal of 1.443 mm.

FIGS. 3A to 3C illustrate plots of lens sag and slope versus radialaperture for lens surfaces A, B, and E of FIG. 2.

As can be seen in FIG. 3A, surface A is a gullwing surface, i.e., has aconvex apex and a concave edge. For surface A, the clear aperture is1.159 mm and the SAG over the clear aperture is 0.008 mm (8 μm).

As can be seen in FIG. 3B, surface B is a convex surface. For surface B,the clear aperture is 1.433 mm and the SAG over the clear aperture is0.042 mm (42 μm).

As can be seen in FIG. 3C, surface E is a convex surface. For surface E,the clear aperture is 1.613 mm and the SAG over the clear aperture is0.071 mm (71 μm).

Thus, for the imaging lens 200, the maximum clear aperture is 1.613 mm,i.e., less than 30% greater than the sensor image diagonal (or the imagecircle), and the maximum SAG is 71 μm, i.e., less than 100 μm.

Further, by having small SAGs, if a starting thickness, i.e., beforeforming the lens surface, of the optical elements 210, 220 is the same,then the center thickness of the optical elements 210, 220 may be within15% of one another. In this particular example, the optical element 210has a center thickness of 0.68 mm and the optical element 220 has acenter thickness of 0.69 mm. For example, when made on a wafer level, astarting thickness of substrates used to create the optical elements210, 220, may be between 0.5 mm and 1.5 mm, e.g., 0.5 mm to 1.0 mm, withthis particular example having a starting thickness of 0.7 mm. Using thesame or standard substrate thickness, particularly thinner substrates,may reduce cost.

Further, in this particular example, the second optical element 220 iscloser to the cover glass 132 than to the first optical element 210,with a difference between these distances, i.e., B to E and F to 132,being less than 50%. In this particular example, the optical tracklength is 3 and a ratio of the optical track length to the imagediagonal of the sensor is less than 2.5.

FIG. 4 illustrates a schematic side view of an imaging capturing system350 in the LWIR waveband in accordance with an embodiment. Asillustrated in FIG. 4, the image capturing system 350 includes animaging lens 300 and the sensor 230.

The imaging lens 300 may include a first optical element 310 and asecond optical element 320. In the schematic illustration of FIG. 4, aspacer (which would include surfaces C and D, see FIG. 6) between thefirst optical element 310 and the second optical element 320 has beenomitted for clarity.

In this particular embodiment, three surfaces, here surfaces A, B, andE, have optical power therein. One, two, or all three surfaces may beaspheric The imaging lens 300 may also include the aperture stop 302,which may have the same configuration/properties noted above foraperture stop 102. The f-number for the imaging lens 300 may be lessthan 1.1.

If the material used for one or both optical elements 310, 320 presentschromatic dispersion over an operational waveband or if the imaging lens300 otherwise requires correction, a diffractive element 304 may beprovided on one or more of the surfaces A, B, E, or F. For example, thediffractive element 304 may be on the surface having the most opticalpower, here, surface E.

For the imaging lens 300, surface A is a gullwing surface having a clearaperture of 1.167 mm and SAG over the clear aperture of 0.017 mm (17μm); surface B is a convex surface having a clear aperture of 1.398 mmand SAG over the clear aperture is 0.039 mm (39 μm); surface E is agullwing surface having a clear aperture of 1.444 mm and SAG over theclear aperture is 0.046 mm (46 μm).

Thus, for the imaging lens 300, the maximum clear aperture is 1.444 mm,i.e., less than 30% greater than the sensor image diagonal (or the imagecircle), and the maximum SAG is 46 μm, i.e., less than 100 μm. Further,in this particular example, the second optical element 320 is closer tothe cover glass 132 than to the first optical element 310, with adifference between these distances, i.e., B to E and F to 132, beinggreater than 50%.

FIG. 5 illustrates a schematic side view of an imaging capturing system450 in the LWIR waveband in accordance with an embodiment. Asillustrated in FIG. 5, the image capturing system 450 includes animaging lens 400 and the sensor 130. The image capturing system 450 isdesigned for a longer optical track length than the embodiments of FIGS.2 and 4, so the imaging lens 400 is of a slightly larger scale, with athickness of the first optical element 410 being 1.019 mm and athickness of the second optical element 420 being 1.488 mm.

The imaging lens 400 may include a first optical element 410 and asecond optical element 420. In the schematic illustration of FIG. 5, aspacer (which would include surfaces C and D, see FIG. 6) between thefirst optical element 410 and the second optical element 420 has beenomitted for clarity.

In this particular embodiment, three surfaces, here surfaces A, B, andE, have optical power therein. One, two, or all three surfaces may beaspheric. The imaging lens 400 may also include the aperture stop 402,which may have the same configuration/properties noted above foraperture stop 102. The f-number for the imaging lens 400 may be lessthan 1.1.

If the material used for one or both optical elements 410, 420 presentschromatic dispersion over an operational waveband or if the imaging lens400 otherwise requires correction, a diffractive element 404 may beprovided on one or more of the surfaces A, B, E, or F. For example, thediffractive element 404 may be on the surface having the most opticalpower, here, surface A.

For the imaging lens 400, surface A is a gullwing surface having a clearaperture of 1.423 mm and SAG over the clear aperture of 0.017 mm (17μm); surface B is a convex surface having a clear aperture of 1.716 mmand SAG over the clear aperture is 0.049 mm (49 μm); surface E is aconvex surface having a clear aperture of 1.750 mm and SAG over theclear aperture is 0.054 mm (54 μm).

Thus, for the imaging lens 400, the maximum clear aperture is 1.75 mm,i.e., less than 30% greater than the sensor image diagonal (or than theimage circle), and the maximum SAG is 54 μm, i.e., less than 100 μm.Further, in this particular example, the second optical element 420 iscloser to the cover glass 132 than to the first optical element 410,with a difference between these distances, i.e., B to E and F to 132,being greater than 50%.

Any of the imaging lenses 100, 200, 300, 400 discussed above may beprovided in a barrel assembly 550, as illustrated in FIG. 6. Inparticular, the barrel assembly 550 may be a threaded barrel assemblysuch that a distance between an imaging lens 500 housed therein and thesensor 130, i.e., along the z-axis, may be altered. As illustratedtherein, the imaging lens 500 may include a first optical element 510and a second optical element 520 separated by a spacer 515 providing anair gap between surfaces B and E. The surfaces B and E may includerelative planar portions 512, 522, i.e., flat regions, in a peripherythereof to facilitate securing of the spacer 515 thereto.

Method of Making

One or both of optical elements noted above may be silicon. Any one,two, or all of the lens surfaces noted above may be made using, e.g.,the stamp and transfer technique disclosed in U.S. Pat. No. 6,027,595,which is hereby incorporated by reference in its entirety. As notedtherein, these surfaces may be created on the wafer level, i.e., aplurality of these surfaces may be replicated and transferred to a wafersimultaneously and later singulated to realize individual opticalelements. Depending on the material of the optical element, othertechniques for forming one or more of the lens surfaces may includediamond turning or molding, e.g., high temperature molding.

In addition to fabrication of surfaces on a wafer level, as disclosed inU.S. Pat. No. 6,096,155, which is hereby incorporated by reference inits entirety, two or more wafers, each having a plurality of opticalelements thereon may be secured together along the z-direction beforesingulation, such that individual optical systems each have an opticalelement from each wafer. A spacer wafer may be provided between the twowafers having the optical elements thereon. Alternatively, one of thewafers having optical elements thereon and the spacer wafer may besecured and singulated, and then secured to another wafer having opticalelements thereon, or one of the wafers having optical elements thereonmay have spacers die bonded thereon and then secured to the other waferhaving optical elements thereon.

Devices Incorporating LWIR Imaging Lens

FIG. 7 illustrates a perspective view of a computer 680 having an LWIRimaging system 600 integrated therein. FIG. 8 illustrates a front andside view of a mobile telephone 690 having the LWIR imaging system 600integrated therein. Of course, the LWIR imaging system 600 may beintegrated at other locations and with other electronic devices, e.g.,mobile devices, entertainment systems, standalone thermal imagers, andso forth, other than those shown. The LWIR imaging system 600 may be anyof those noted above.

As used herein, the term “and/or” includes any and all combinations ofone or more of the associated listed items. Further, although terms suchas “first,” “second,” “third,” etc., may be used herein to describevarious elements, components, regions, layers and/or sections, theseelements, components, regions, layers and/or sections should not belimited by these terms. These terms are only used to distinguish oneelement, component, region, layer and/or section from another. Thus, afirst element, component, region, layer and/or section could be termed asecond element, component, region, layer and/or section withoutdeparting from the teachings of the embodiments described herein.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” etc., may be used herein for ease of description to describethe relationship of one element or feature to another element(s) orfeature(s), as illustrated in the figures. It will be understood thatthe spatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. For example, if the device in thefigures is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the exemplary term “below” can encompass both anorientation of above and below. The device may be otherwise oriented(rotated 90 degrees or at other orientations) and the spatially relativedescriptors used herein interpreted accordingly.

As used herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises,”“comprising,” “includes,” and “including” specify the presence of statedfeatures, integers, steps, operations, elements, components, etc., butdo not preclude the presence or addition thereto of one or more otherfeatures, integers, steps, operations, elements, components, groups,etc.

Embodiments of the present invention have been disclosed herein and,although specific terms are employed, they are used and are to beinterpreted in a generic and descriptive sense only and not for purposeof limitation. In some instances, as would be apparent to one ofordinary skill in the art as of the filing of the present application,features, characteristics, and/or elements described in connection witha particular embodiment may be used singly or in combination withfeatures, characteristics, and/or elements described in connection withother embodiments unless otherwise specifically indicated. Accordingly,it will be understood by those of ordinary skill in the art that variouschanges in form and details may be made without departing from thespirit and scope of the present invention as set forth in the followingclaims.

What is claimed is:
 1. An imaging lens for use with an operationalwaveband over any subset of 7.5-13.5 μm, the imaging lens comprising: afirst silicon optical element, the first silicon optical element havinga front surface and a rear surface; and a second silicon opticalelement, the second silicon optical element having a front surface and arear surface, the front surface of the second silicon optical elementfacing the rear surface of the first silicon optical element, wherein atleast two surfaces of the first and second optical elements areoptically powered surfaces, a center thickness of each of the first andsecond optical elements is greater than 500 μm and less than 1500 μm,and the first and second optical elements are the only elements havingoptically powered surfaces in the imaging lens.
 2. The imaging lens asclaimed in claim 1, wherein three surfaces of the first and secondoptical elements are optically powered surfaces.
 3. The imaging lens asclaimed in claim 2, wherein the optically powered surfaces are the frontand rear surfaces of the first optical element and the front surface ofthe second optical element.
 4. The imaging lens as claimed in claim 3,wherein all three of the optically powered surfaces are aspheric.
 5. Theimaging lens as claimed in claim 1, wherein each optically poweredsurface has a maximum sag height difference across the clear aperture ofless than 100 μm.
 6. The imaging lens as claimed in claim 5, whereineach optically powered surface has a maximum sag height differenceacross the clear aperture of 50 μm or less.
 7. The imaging lens asclaimed in claim 1, wherein one of the optically powered surfaces isaspheric.
 8. The imaging lens as claimed in claim 1, wherein two of theoptically powered surfaces are aspheric.
 9. The imaging lens as claimedin claim 4, wherein the F-number of the imaging lens is less than 1.1.10. The imaging lens as claimed in claim 1, wherein center thicknessesof the first and second optical elements are within 15% of one another.11. The imaging lens as claimed in claim 1, wherein the F-number of theimaging lens is less than 1.1.
 12. The imaging lens as claimed in claim1, wherein the imaging lens is used in at non-cryogenic temperatures.